The present invention relates to an interferometer sensor and a method of manufacturing an interferometer sensor. In particular, the present invention relates to a method for forming a polymer film Fabry-Perot interferometer sensor.
A known interferometer comprises a polymer interferometer film, the deflection or compression of which, by a signal for analysis, modulates multiple reflections of an incident optical interrogation signal. For example, a known optical fibre interferometer using a polymer film comprises an optical fibre having a cleaved end and a polymer sensing film butted against the cleaved end. Two opposite faces of the polymer film provide the two reflecting surfaces of the interferometer. Light is introduced to the optical fibre and any external change that causes a variation in the optical thickness of the is sensor film can be detected, since modulation of the thickness of the polymer film influences the output of the interferometer sensor. The external changes could include acoustic waves, quasi-static pressure and temperature variations or thermal waves caused by transient heating.
Conventionally, a disc of PET (polyethylene terepthalate) may be used as the polymer film. The disc is cut from a larger piece of the PET and adhered to the cleaved end of the optical fibre using a conventional adhesive agent. However, high uniformity in the thickness of the polymer film is required, and any irregularities in the surfaces of the PET or a any lack of uniformity in the thickness of the PET can adversely affect the operation of the interferometer. The birefringence of the PET film also has adverse effects on the sensor operation. In addition, the process of cutting out and attaching the PET film to the cleaved end of the optical fibre is complex and time consuming.
Furthermore, the applicant has recognised that the use of an adhesive agent disposed between the inner surface of the PET disc and the cleaved end of the optical fibre can further affect the operation of the interferometer. In particular, due to its finite thickness, the adhesive agent can itself act as an additional interferometer film.
According to a first aspect of the present invention, there is provided a method of forming an interferometer film for an interferometer sensor comprising the step of forming a polymer layer of substantially uniform thickness directly on an interferometer substrate, the layer forming the interferometer film, wherein the polymer layer is deposited by polymerisation of a gas of monomer particles including a para-xylylene.
Since the interferometer film is formed directly onto the surface of the interferometer substrate, there is improved conformity between the two surfaces at the interface between the polymer layer and the substrate. Furthermore, improved uniformity in the thickness of the film can be achieved. Since no layer of adhesive is required to fix the interferometer film to the substrate, adverse interference effects from an adhesive layer are avoided.
Para-xylylene compounds are particularly effective in this application. They offer uniformity and completeness of coverage in addition to good physical, electrical, chemical, mechanical and barrier properties. Furthermore, no solvents are released during the coating process and the process is thus not affected by volatile organic compound (VOC) regulatory restrictions. In addition, the encapsulation provided by para-xylylene is excellent, being free of pin-holes in coatings as thin as 1 xcexcm.
The coating is formed by the condensation of the monomer gas, preferably under a weak vacuum, molecule by molecule. A comparatively weak vacuum (for example 10 to 20 Pa) means that the monomer gas has a mean free path of around 0.1 cm, enabling the coating to form uniformly on all surfaces, in contrast to other vapour deposition methods such as thermal evaporation or sputtering in which the deposition is line-of-sight. The method of deposition of this aspect of the invention provides good uniformity of thickness, which is an essential requirement for Fabry-Perot sensing interferometers, as they require interferometrically flat surfaces.
The use of a weak vacuum also reduces the likelihood of outgassing of the substrate or other components in deposition chamber.
A wide range of thicknesses of the polymer film can be achieved, for example from 0.025 microns to 75 microns, with high thickness tolerance due to the controllable nature of the process.
Deposition preferably takes place at room temperature, avoiding damage to heat sensitive substrates such as low melting point polymers (eg poly methyl methacylate) or the optical coating on the substrate. It also avoids thermal cycling-induced stresses during deposition process which could damage the substrate or the optical coating on the substrate. Thermally-induced stresses could also lead to the film becoming birefringent which impairs sensor performance.
The method may comprise, prior to the step of polymerising, the step of forming a gas of monomer particles in a first chamber at a first pressure and a first temperature and coupling the gas of monomer particles to a deposition chamber. The substrate is placed in the deposition chamber and, at a second pressure (preferably the weak vacuum) and second temperature (preferably the ambient temperature), monomer particles polymerise on the substrate.
According to a second aspect of the present invention, there is provided an interferometer sensor comprising an interferometer substrate and a parylene polymer film of substantially uniform thickness, in which the parylene film is formed directly on the interferometer substrate.
The invention also provides medical analysis equipment having an interferometer sensor assembly comprising:
an interferometer sensor of the invention;
an interrogation source to provide an interrogation signal to the sensor; and a detector to detect signals received from the sensor.